1. Field of the Invention
This invention pertains to the field of liquid dispensing equipment. More particularly, it pertains to design of a pump that employs a novel method of pumping or extruding a fluid. The pump design is capable of accomplishing this task without the use of valves or redirection of fluid through ancillary pathways. Both cores occupy a polymer shell; each core is inserted from opposite ends, a reservoir and a prime chamber are installed on the topside of the polymer shell. The exit perpendicular passage is directly below and provides for connection of a nozzle or other passage for extrusion of the fluid.
2. Description of the Prior Art
At present there are four general types of pumps used to underfill electronic devices with viscous liquid: (1) A screw or auger type pump comprised of a rotating helix or thread turning inside a cylindrical chamber, the liquid is pumped as a result of shear of the fluid, forward pressure builds as a function of the cosine of the helix angle. (2) An air over type pump, constructed using a cylindrical cavity or syringe, utilizes a column of fluid or reservoir with a follower or concave disc placed on top, air pressure creates the force to move the liquid by acting on the surface area of the follower, toggling the air on and off starts and stops the flow. (3) A jet type pump constructed from a poppet valve, the poppet valve is a rod with a spherical end that moves in a translational fashion over a puddle of fluid, a carbide orifice below the puddle provides the path for a minute quantity of liquid to be expelled as the spherical end impacts the puddle. (4) A positive displacement type pump moves a column of liquid by displacement of a volume of fluid in the chamber equal to the quantity extruded through the exit port, the rate of flow through the exit port is a function of the speed the piston advances multiplied by the volume displaced.
Pumps made for dispensing of viscous fluids by positive displacement require a provision in the design to accomplish the three distinct tasks to ready the pump for its intended function. The three machine states are prime, refill and dispense.
The first state, prime, is performed apriori of dispensing the fluid. It is always required of this type of pump to fill the pump cavity with fluid that is free of air bubbles. Precision dispensing of fluid using the positive displacement technique is susceptible to error in the dispensed volume from air entrapped in the fluid. The problem has a negative impact on the pump repeatability due to the inherent compressibility of air in contrast to the relative incompressibility of most liquids. Two techniques commonly used to rid the pump of this nuisance variable are: Pushing the fluid through the cavity until all air is displaced and the entire volume is homogeneous with respect to fluid, the second is pulling the liquid through the cavity by use of vacuum to achieve the same. Both techniques require this task to be accomplished until all air is dispelled; usually this requires visual inspection of the fluid exiting the chamber via a clear tube. All pumps available for use in the semiconductor industry today discard primed fluid as waste; this practice is expensive due to the high cost of the fluid. Sensors or cameras can be used to detect the presence of oxygen or bubbles; it is possible to automate the process.
The second state, refill, is accomplished immediately after priming the pump and after the fluid in the chamber is depleted at the conclusion of a dispense. Refill of the chamber occurs when the piston in the pump is retracted at the same rate as liquid from the fluid reservoir advances. Fluid from the reservoir is pushed forward by gravity, air pressure or mechanical means, simultaneously filling the cavity, preventing entrapment of air in the liquid. Cavitation occurs when a liquid contains air or other compressible gas as a result of not advancing to fill the volume as rapidly as the piston retracts. If this happens, the pump must be primed again or accuracy and repeatability of the volume dispensed will be poor. Solutions used in semiconductor applications are expensive and pumps with no capacity to reuse the fluid expelled from the prime state are costly to operate.
The third state, dispense, occurs after the pump has been primed, refilled and the piston is at the top of the cylinder poised to push the column of fluid through the exit port. The exit port provides a mechanical connection for a nozzle or attachment of another passage for extrusion of the fluid.
The current trend in the industry is to construct and design pumps of the positive displacement type using one piston for each fluid cavity. Pumps are generally mounted in the upright configuration; the chamber attitude is perpendicular to the surface of the earth. Some manufacturers employ the concept of dual chambers side by side with one piston per cavity. This method is used to mask refill time, one chamber can dispense while the other refills.